When high‑energy ions hit lunar dust, they leave behind separated positive and negative charges that can persist for long periods. This is clear from studies of how energetic‑particle irradiation produces persistent charge separation within lunar regolith. However, the magnitude of the internal electric fields that models predict depends on how the initial microscopic interactions are translated into a retained charge that remains after the particles stop. To connect the ion bombardment to a macroscopic charging model, researchers use the average retained‑charge response. This quantity is defined as the average signed excess charge per unit depth per incident particle and is measured or calculated at a specific evaluation time. It captures how much net charge stays in a thin depth layer of regolith after an ion of species s and energy E hits the surface. Different modelers treat the depth distribution of charge in distinct ways. One group approximates the energized region as two idealised planar charge layers separated by an internal electric field, treating the subsurface as a leaky parallel‑plate capacitor. Another group uses a continuous depth‑dependent volumetric charge density that changes with depth, coupled with Gauss’s law. The choice between these approaches affects the predicted electric fields and surface‑charging dynamics. Laboratory experiments that measure electron emission from regolith simulants only track electrons that escape the sample into a detector, not the internal re‑absorption or the depth‑dependent retained charge. Therefore, these experiments provide limited information for subsurface charging on the Moon. Likewise, no experiment yet has measured the depth‑dependent retained‑charge profile under solar‑energetic‑particle or galactic‑cosmic‑ray irradiation conditions relevant to the Moon, leaving the full retained‑charge profile unconstrained. The key takeaway is that while energetic particles can produce lasting electric fields inside our nearest planetary neighbour, predicting the exact strength of those fields requires detailed knowledge of how particles deposit energy and how the resulting excess charge is retained at different depths in lunar soil.